The present invention relates to a method and an apparatus for detecting decrease in tire air-pressure as well as to a program for judging decompression of a tire. More particularly, it relates to a method and an apparatus for detecting decrease in tire air-pressure as well as to a program for judging decompression of a tire with which it is possible to accurately detect decrease in air-pressure of a tire.
An apparatus for detecting decrease in tire air-pressure (DWS) conventionally employs a theory that a rotational angular velocity of a decompressed tire is increased when compared to remaining normal tires owing to a decrease in outer diameter (dynamic load radius of the tire) from that of a tire of normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires,DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is employed as a judged value DEL (Japanese Unexamined Patent Publication No. 305011/1988). Here, F1 to F4 denote rotational angular velocities of a front left tire, a front right tire, a rear left tire and a rear right tire, respectively.
Since changes in the dynamic load radii of tires owing to decompression vary depending types of tires or position of mounting the same, it is necessary to detect which of the tire is decompressed for making alarm to be issued when a specified decompression rate has been reached. However, such determination of a decompressed wheel is technically difficult. Accordingly, in a method for judging decompression of a tire on the basis of changes in dynamic load radii of tires, a considerably large tolerance of, for instance, ±10% is provided for a decompression rate for alarm of, for instance, 30%, since decompression rates for issuing alarm fluctuate depending on types of tires and influences of mounting positions. However, since dynamic load radii of tires fluctuate, in addition to influences of air-pressure or types of tires, also through influences of load, slip angle, camper angle, driving force or braking force, erroneous alarm might still be issued in spite of normal air-pressure even when providing a large tolerance for the decompression rate for alarm.
There is known a method for improving the accuracy of alarm as a method for solving such a problem wherein the level of alarm is made constant regardless of the position of the tires by correcting the judged value for alarm by using position sensitivity coefficients depending on mounting positions of tires (Japanese Unexamined Patent Publication No. 151919/1998).
In the prior art method, a sensitivity coefficient is multiplied to wheel speeds of respective tires for making the decompression sensitivity constant (uniform) so that alarm might be issued at a constant decompression rate without determining a position of a decompressed wheel. However, there is a risk that shifts in judged values are caused at the time of performing turning movements so that erroneous alarm is issued by multiplying a sensitivity coefficient other than “1” to the wheel speeds.